The present disclosure is generally directed to measuring the anatomical properties of trees, and in particular relates to a method for characterizing the density and cross-section morphology of trees.
The health, conservation and restoration of national forest lands in the United States often requires that the stands be selectively thinned by removing small-diameter trees, many of which exhibit suppressed growth. Suppressed-growth trees make up a significant component of what are known as small-diameter trees, which are trees whose diameters are significantly smaller than expected for mature trees of a given species. To offset the high cost of forest thinning operations, it is desirable to find high value, large volume uses for the removed small-diameter trees. One potential approach would be to use the trees as a source of fiber in paper making processes. Unfortunately, the fiber of small-diameter trees is not conventionally accepted as a reliable source by the pulp and paper industry. While the basic material properties of wood from small-diameter trees indeed differ from those of traditional pulpwood, the industry lacks a proper understanding of the differences and whether they are relevant to the viability of the wood in paper production.
It is well-known that the morphology of wood tracheids, for instance the dimensions of tracheid wall thickness and lumen diameter, and their density, affect the processing and properties of both lumber and paper. Because wood species' anatomy and growth rate dictate the fiber morphology, the understanding and characterization of the differences in fiber morphology of wood from small-diameter trees allows one to predict the variability and thus the viability of the wood in end-use applications such as paper production. For instance, it is believed that greater uniformity in certain anatomical properties between earlywood and latewood could result in an acceptably minimal amount of damage to the wood fibers (or tracheids) in thermomechanical pulping.
Earlywood, also known as springwood, refers to wood tracheids produced by a tree at the beginning of the tree's growing season, and makes up the light-color section in the tree's annular (or growth) ring compared to the tracheids produced during the middle to end of the growing season. Latewood, also known as summerwood, refers to wood tracheids produced by a tree at the end of the tree's growing season, and make up the darker section of the tree's annular ring compared to the cells produced during the middle of the growing season. Earlywood tracheids form the inner portion of the annular ring and are typically thinner walled than latewood, which forms the outer portion of the annular ring.
Conventional methods to analyze the anatomical properties of wood have achieved only limited success.
For instance, one such method involves using near infrared (NIR) spectroscopy techniques. NIR relies on multivariate calibration and requires an established anatomical database specific for each tree species of interest. Unfortunately, the diversity of small-diameter trees makes a useful database difficult to obtain, and such a database is currently unavailable.
Another method involves X-ray densitometry and optical microscopy. While the analysis is capable of quick measurements with good statistical significance, the relatively low spatial resolution used, coupled with line-of-sight measurements through curved growth rings, makes it unsuitable for measurements of density of suppressed-growth trees whose annual rings have a width less than several hundred microns. For instance, if the tracheid layers of a given ring are small compared to the resolution of the scan, individual density measurements can include both earlywood and latewood portions, thus producing results that erroneously indicate a high degree of uniformity between the earlywood and latewood.
Furthermore, the X-ray densitometry and optical microscopy method of calculating wall thickness from measured density and diameter is unreliable for the tracheids of suppressed-growth trees, which are often irregular in shape and substantially thicker in the tangential direction than in the radial direction. Furthermore, the costs of such measurement systems are often prohibitive, thus limiting the availability and usage of the measurement systems throughout the forestry industry. In fact, one such measurement system in existence today is available at only three locations world-wide.
Another conventional system uses confocal laser scanning fluorescence microscopy to produce images of a wood sample. Tracheid geometry was then compared with resulting pulp fiber geometry, however this system does not characterize the anatomical properties of a wood sample from the pith to bark. Furthermore, such systems are labor-intensive. It is therefore unlikely that such a system would be capable of repeat measurements rapidly enough to characterize a tree.
What is therefore needed is a method for reliably determining the anatomical properties of trees and in particular of suppressed-growth trees.